As is well known in the gas turbine engine field of technology a great effort has been devoted to improve engine operating performance by attempting to hold the clearance of the gap between the outer air seal and the tip of the turbine blade at a minimum during the full range of the engine's operating envelope. What has developed over the years is a host of inventions and concepts that have through active and passive clearance controls attempted to achieve this end. To some degree, many of these inventions and concepts have proven to be successful, but owing to the increasing demands on engine and aircraft performance, the problem has become increasingly more difficult. The solution to the problem is also predicated on the type of aircraft/engine being designed and to its particular mission. What may be satisfactory for say, aircraft used by commercial airlines is typically not satisfactory for military aircraft, particularly those designed to be in the fighter class.
For example, in engines designed for use in aircraft used for commercial applications, such as those used in revenue service, the active clearance controls have been proven to be fairly successful. An example of an active clearance control that has met considerable commercial success is exemplified in U.S. Pat. No. 4,069,662 granted to Redinger et al on Jan. 24, 1979 and assigned to the assignee common with this patent application. This type of control judiciously impinges air on the engine's outer case in proximity to the turbine rotor in order to shrink the case at predetermined times during its operating envelope and hence, position the outer air seal closer to the tips of the turbine or compressor blades so as to reduce the gap.
In contrast to the active clearance control design philosophy, the passive clearance control utilizes a continuous means to effectuate the control of these clearances. For example, one such system continuously impinges cooling air on the outer engine case in proximity to the rotor blades to limit the rate of expansion of the outer case subjected to elevated temperatures in order to hold clearances to a minimum. In short the "active" type of control requires a control system that responds to an input and applies hot or cold air or mechanical means in order to effectuate control of the clearances. The "passive" type of clearance control doesn't require a control system and is in a quiescent state at all times.
This invention contemplates a passive type of clearance control that utilizes the discharge air used for internally cooling of the turbine blades for controlling the effective clearances between the tip of the turbine blade and the outer air seal. Means are disclosed for imposing an aerodynamic seal in the gap between the outer air seal and tips of the blades so as to minimize flow therein and hence, reduce the flow from the high pressure side of blade to the low pressure side thereby increasing the efficiency of the turbine. Essentially, discharge holes drilled into the tip of the turbine blade near the intersection of the tip surface with the airfoil pressure side surface discharge a jet of cooling air into the gap between the outer air seal and the tips of the blade. In certain application intersecting holes may be employed. One of the holes communicates with the internal coolant passage adjacent the pressure side and the other hole communicates with the internal coolant passage adjacent the suction side. The holes intersect to affect the velocity (momentum) and angle of the spent cooling air discharging from the slot formed from the two drilled holes.
Tip holes and intersecting holes in air cooled turbine blades are known in the art. For example, U.S. Pat. Nos. 4,540,339 granted to Horvath on Sep. 10, 1985 and 5,062,768 granted to Marriage on Nov. 5, 1991 each disclose intersecting holes located at the tip of the turbine blade. In one instance, the flow egressing from these discharge holes serves to scrub the side wall surfaces of the squealer tip cap and in the other instance the intersecting holes decrease the likelihood of contamination and hole blockage. While these patent do not address the use of these holes for aerodynamic sealing, there are known apparatus that utilize tip holes for aerodynamic sealing.
As disclosed in the prior art, there is a concern that foreign matter and particularly scrapings occasioned by blade tip rubs migrate to the discharge ports in the tips of the blade adversely affecting the flow discharging therefrom. This problem has become more acerbated because the blade tips are currently being coated with abrasive material which has the propensity of having particles becoming dislodged in a blade rub. Obviously this increases the possibility of clogging these discharge ports which not only has an impact on the cooling effectiveness of the cooling passage and the discharging flow, but also in a tip sealing configuration, the plugged up holes adversely affects the tip sealing capabilities and hence, degrade the turbine's efficiency.